The present invention relates to a switching power source applicable to various kinds of electronic apparatuses as a stabilizing power source and, more particularly, to a switching power source capable of stabilizing an output voltage with a high transform efficiency even when an input voltage varies over a broad range.
A switching power source is extensively used with electric and electronic apparatuses since it is small size, light weight, and highly efficient. A switching power source has customarily been made up a transformer, a main transistor, and a control circuit. The transformer has a primary winding and a control winding on the primary side and an output winding on the secondary side. The main transistor is connected between the primary side of the transformer and an input power source. The control circuit controls the base current of the main transistor. In operation, when the main transistor is turned off, an output current is produced while the negative voltage on the control winding of the transformer is rectified to generate a negative voltage. The negative voltage is compared with a reference voltage. The base current of the main transistor is so controlled as to equalize the negative voltage and the reference voltage, whereby the output voltage is stabilized. Since a switching circuit generally has to operate without errors over a range of input voltages of 80 V to 270 V, constants insuring the necessary base current even when the input voltage is 80 V are selected.
In the conventional switching power source described above, a current proportional to the input voltage flows from the control winding to the base of the main transistor. This gives rise to a problem that when the input voltage is 270 V, a current more than three times as great as the necessary base current flows. The current more than necessary is simply consumed by the control circuit and base resistance, degrading the efficiency of the power source.